Typically, ailerons and an elevator control the flight direction of airplanes. Ailerons are normally a part of the trailing edge, the aft part of the wing, which is hinged so it can tilt up and down. When the aileron is tilted down it alters the shape of the wing and in effect increases the incidence angle and the angle of attack and thereby also the lift on that wing. When the aileron is tilted down on one wing it is always tilted up on the opposite wing and thereby reducing the lift on this wing.
The incidence angle is the angle between the cord line of the wing and the longitudinal axis of the aircraft itself. The angle of attack is on the other hand defined as the angle between the cord line and the direction of the airflow. If we change the incidence angle and keep everything else unchanged, it can be appreciated that the angle of attack is changed by the same amount. However, changing the attitude of the aircraft by e.g. pulling the nose up, will change the angle of attack while the incidence angle remains unchanged.
The ailerons control the roll, the banking, of the airplane while the elevator controls the pitch, the up-down direction of flight. The elevator is typically placed at the trailing edge of the stabilizer at the rear end of the airplane and by tilting it up or down it alters the lift force on the stabilizer and thereby controls the up and down direction.
To control the flight direction; the ailerons are used to bank the airplane sideways and by applying a little up-elevator the airplane performs a turn while it keeps its height in the air.
For a slow flying aircraft the ailerons can have less effect and especially on single propeller airplanes it is possible to instead use the rudder to control the flight direction. The rudder is placed vertically at the tail of the airplane and controls the yaw.
Single propeller airplanes normally have the propeller placed in the front, creating a fast airflow over the stabilizer, elevator and rudder. Twin-engine airplanes, very slow flying gliders or flapping wing aircrafts like ornithopters, however, lack the additional airflow over the stabilizers and rudder that single propeller aircrafts normally have. For these kinds of aircrafts it can be more difficult to get a good directional control.
One way of overcoming this problem is in the case of a twin-engine airplane to use differential thrust. Each of the two motors, jet engines or propellers which typically are placed one on each wing, can be controlled individually. By increasing the speed of one motor and reducing the speed of the opposite motor the flight direction can be controlled. This is a well-known way of controlling a twin-engine airplane and it is described in e.g. U.S. Pat. No. 6,612,893.
In the case of ornithopters the forward thrust is produced by the flapping wings and not by propellers. If the ornithopter in addition flies slowly, a normal rudder at the back of the aircraft has reduced effect. One way of trying to solve this problem is to make the whole tail movable. This solution is shown in e.g. U.S. Pat. No. 6,550,716. Here the whole tail is hinged and controlled by servos. This solution is believed to be both fragile and complicated.
A simpler way of controlling slow flying small aircrafts, like remotely controlled toy airplanes or slow flying ornithopters is to use a small vertically placed propeller instead of the rudder at the rear end of the aircraft. This method is described in US patent application US 20040169485. The small propeller can blow air to either left or right and thereby pushes the tail sideways to control the flight direction. However, when the aircraft turns e.g. to the left it normally also banks or rolls over to the left. In this position the tail is pushed up by the blowing tail propeller and the effect of this is almost like having a down-elevator action forcing the aircraft into a downwardly turn instead of a gentle turn where the height is kept. This tendency makes it more difficult to perform tight maneuvers with this system.
Especially for slowly flying aircraft with high angles of attack and for flapping wing aircrafts the existing systems have limitations. Some of the ways for controlling the flight direction described above are both innovative and simple but it is believed that an even simpler and better system is possible.